Light-Absorbing Structure and Methods of Making

ABSTRACT

A critically coupled optical resonator absorbs greater than 95% of incident light of the critical wavelength with an absorber layer less than 10 nm thick.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberMDA972-00-1-0023, awarded by DARPA. The government has certain rights inthe invention.

TECHNICAL FIELD

This invention relates to light-absorbing structures and methods ofmaking the structures.

BACKGROUND

Light-emitting devices can be used, for example, in displays (e.g.,flat-panel displays), screens (e.g., computer screens), and other itemsthat require illumination. Accordingly, the brightness of thelight-emitting device is one important feature of the device. Also, lowoperating voltages and high efficiencies can improve the viability ofproducing emissive devices.

Light-emitting devices can release photons in response to excitation ofan active component of the device. Emission can be stimulated byapplying a voltage across the active component (e.g., anelectroluminescent component) of the device. The electroluminescentcomponent can be a polymer, such as a conjugated organic polymer or apolymer containing electroluminescent moieties or layers of organicmolecules. Typically, the emission can occur by radiative recombinationof an excited charge between layers of a device. The emitted light hasan emission profile that includes a maximum emission wavelength, and anemission intensity, measured in luminance (candelas/square meter (cd/m²)or power flux (W/m²)). The emission profile, and other physicalcharacteristics of the device, can be altered by the electronicstructure (e.g., energy gaps) of the material. For example, thebrightness, range of color, efficiency, operating voltage, and operatinghalf-lives of light-emitting devices can vary based on the structure ofthe device.

SUMMARY

A high oscillator strength thin film can be applied to a surface. Thefilm can have an absorption coefficient greater than 10⁵ cm⁻¹, forexample, 10⁶ cm⁻¹ or larger. The films can be formed by adsorption intolayered structures of charged species with strong dipole-dipoleinteractions between species. The films can be built by adsorption ofspecies with alternating charge on a solid substrate such as glass,silicon, a polymer surface, or a previous polymer film disposed on asubstrate, etc.

The high absorption coefficient can arise from the interaction ofdipoles in a plane perpendicular to the probe direction. The processused to form these films can allow for strong dipole interactions withinthe adsorbed layer. Additionally, the process can allow for precisedeposition of a single physical layer of the dipole-dipole-interactingabsorbing species. As a result, the dipole-dipole interactions in theplane of absorbing species perpendicular to the probe direction canprovide a high absorption constant in the thin film.

The high oscillator strength film can be an element of a criticallycoupled resonator (CCR). The CCR can include a reflective element (i.e.,a structure capable of reflecting light of a desired wavelength, such asa mirror or dielectric Bragg reflector) optically coupled to the film.Optical coupling of the high oscillator strength film and the reflectiveelement can give rise to critical coupling, in which greater than 90% oflight having a critical wavelength is absorbed. The CCR can absorbgreater than 90%, greater than 95%, or greater than 97% of the light atthe critical wavelength. The critical coupling can be characterized by avery low reflectance at a critical wavelength, where the reflectiveelement has a high reflectance in the absence of the critically coupledabsorber. The CCR can include a top coat.

In one aspect, an optical device includes a light-absorbing film havinga thickness and separated a distance apart from a reflective element.The light-absorbing film is critically coupled to the reflectiveelement. The light-absorbing film can be critically coupled to thereflective element at a temperature above 77 Kelvin. The light-absorbingfilm can be critically-coupled at a wavelength between 250 nm and 2000nm, such as between 250 nm and 400 nm, between 400 nm and 700 nm,between 700 nm and 900 nm, between 900 nm and 1200 nm, or between 1200nm and 2000 nm.

The light-absorbing film can be separated a distance apart from thereflective element by a light-transmitting material. The thickness ofthe light-absorbing film can be less than 80 nm, less than 50 nm, lessthan 25 nm, or less than 10 nm. The light-absorbing film can include alight-absorbing material. The light-absorbing film can include amultiply charged material. The light-absorbing material can include anorganic compound or an inorganic compound. The light-absorbing materialcan include a J-aggregate, which can include a cyanine dye. The multiplycharged material can include a polyelectrolyte. The light-absorbing filmcan include an electrostatic bilayer which includes a first layerincluding a polyelectrolyte and a second layer including alight-absorbing material. The optical device can be arranged on asubstrate. The device can absorb at least 90% or at least 95% of lightat a critical wavelength.

The reflective element can be a dielectric reflector including aninsulator or semiconductor material or can include a metallic mirror.The reflective element can be a dielectric Bragg reflector, composed ofinsulator or semiconductor materials. The reflective element can includea semiconductor material. A dielectric reflector can derive itsreflectivity from interference phenomena associated with the real partof the index of refraction of the reflecting elements. A very reflectivemirror can be constructed from insulating or semiconducting materialsbecause the reflectance is derived from a multitude of interferenceeffects. A dielectric Bragg reflector refers specifically to a mirrorwhere the thickness, d_(i), of the different materials is chosen tosatisfy the Bragg condition discussed in the text.

In another aspect, an optical device includes a light-absorbing filmhaving a thickness of less than 80 nm and an extinction coefficient (K)of at least 1 at a critical wavelength, and being separated a distanceapart from a reflective element by a light-transmitting material,wherein the light-absorbing film is critically coupled to the reflectiveelement. The light-absorbing film can have an extinction coefficient (K)of at least 2, at least 3, at least 4, or at least 5 at a criticalwavelength.

In another aspect, a method of making an optical device includesarranging a light-absorbing film having a thickness a distance apartfrom a reflective element. The distance is selected to critically couplethe light-absorbing film to the reflective element.

The method can include arranging the reflective element on a substrate.Arranging the reflective element on the substrate can include applying ametallic mirror to the substrate, or forming a dielectric reflector onthe substrate. The method can include disposing a light-transmittingmaterial adjacent to the reflective element. Disposing alight-transmitting material can include forming a desired thickness ofthe light-transmitting material.

Arranging the light-absorbing film a distance apart from the reflectiveelement can include applying the light-absorbing film adjacent to thelight-transmitting material. Arranging the light-absorbing film caninclude contacting a surface of the light-transmitting material with amultiply charged material. The multiply charged material can include apolyelectrolyte. Arranging the light-absorbing film can includecontacting a surface of the light-transmitting material with alight-absorbing material.

In another aspect, a method of making an optical device includes forminga thickness of a light-transmitting material adjacent to a reflectiveelement, and forming light-absorbing film having a thickness of lessthan 80 nm and an extinction coefficient (k) of at least 1 at a criticalwavelength adjacent to the thickness of the light-transmitting material.The light-absorbing film is critically coupled to the reflectiveelement.

In another aspect, a device includes a light emitting layer configuredlight of a first wavelength, an absorber or emitter layer configured toreceive the first wavelength emitted from the light emitting layer andtransmit light of a second wavelength, and a dielectric reflectorconfigured to receive the light of the second wavelength and transmitlight out of the device. The light emitting layer can be patterned, theabsorber or emitter layer can be patterned, or both.

In other aspects, a chemical sensor, a light harvesting device, or anoptical switch can include a light-absorbing film having a thickness andseparated a distance apart from a reflective element.

Other features, objects, and advantages will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF TEE DRAWINGS

FIGS. 1A-1B are a schematic depictions of a critically coupledresonator.

FIG. 2 is a graph depicting results of optical measurements oncomponents of a critically coupled resonator and of an assembledcritically coupled resonator.

FIG. 3 is a graph depicting calculated optical properties of a lightabsorbing film.

FIG. 4 is a graph compared measured and calculated optical properties ofoptical devices.

FIG. 5 is a graph depicting calculated optical properties of acritically coupled resonator.

FIGS. 6A-6C are schematic diagrams depicting a device including acritically coupled resonator.

FIG. 7 is a schematic depicting increase of photoluminescence using acritically coupled resonator.

DETAILED DESCRIPTION

Critical coupling occurs when (1) all of the incident optical power istransferred through the front face of the CCR absorber layer and (2) thePoynting vector in the dielectric layers is purely imaginary.Consequently, two boundary conditions must be simultaneously satisfiedto achieve critical coupling: that is, the magnitude of the reflectioncoefficient from air to the absorber layer front face must be zero, andthe magnitude of the reflection coefficient from the absorber layer backface to the dielectric spacer must be unity. The first condition isrealized by impedance matching the CCR with air, and the second bymismatching the impedances across the absorber-spacer interface by aphase difference of ± p/2. The second boundary condition also dictatesthat the Poynting vector is purely real on the absorber layer side ofthe interface.

Thin films having a high oscillator strength (i.e., absorptioncoefficient) can be made by alternately adsorbing two or more materialscapable of non-covalent interaction onto a support or substrate fromsolution, where one material is a light absorbing material. Thenon-covalent interaction can be, for example, an electrostaticinteraction or hydrogen bonding. Selection of appropriate materials andassembly conditions can result in a film where the light absorbingmaterial participates in strong dipole-dipole interactions, favoring ahigh absorption coefficient. The light absorbing material can be a dyecapable of forming a 1-aggregate.

J-aggregates are crystallites of dye in which the transition dipoles ofthe constituent molecules strongly couple to form a collective narrowlinewidth optical transition possessing oscillator strength derived fromall the aggregated molecules. See, e.g., M. Vanburgel, et al., J. Chem.Phys. 102, 20 (1995), which is incorporated by reference in itsentirety.

Layers of light absorbing material, which can be positively ornegatively charged, can be interspersed with layers of an oppositelycharged material. The oppositely charged material can include a multiplycharged species. A multiply charged species can have a plurality ofcharge sites each bearing a partial, single, or multiple charge; or asingle charge site bearing a multiple charge. A polyelectrolyte, forexample, can have a plurality of charge sites each bearing a partial,single, or multiple charge. A polyelectrolyte has a backbone with aplurality of charged functional groups attached to the backbone. Thecharged functional groups attached to the backbone can be exclusivelycationic (as in a polycation), exclusively anionic (as in a polyanion),or be a mixture of cationic groups and anionic groups. A copolymer ofcationic and anionic monomers is an example of a polyelectrolyte havinga mixture of cationic groups and anionic groups. Some polyelectrolytes,such as copolymers, can include both polycationic segments andpolyanionic segments. The copolymer can be, for example, a blockcopolymer, a random copolymer, or other copolymer. A polycation has abackbone with a plurality of positively charged functional groupsattached to the backbone, for example poly(allylamine hydrochloride) orpoly(diallyldimethylammonium chloride). A polyanion has a backbone witha plurality of negatively charged functional groups attached to thebackbone, such as sulfonated polystyrene (SPS), polyacrylic acid, or asalt thereof. Some polyelectrolytes can lose their charge (i.e., becomeelectrically neutral) depending on conditions such as pH. The chargedensity of a polyelectrolyte in aqueous solution can be pH insensitive(i.e., a strong polyelectrolyte) or pH sensitive (i.e., a weakpolyelectrolyte). Without limitation, some exemplary polyelectrolytesare poly diallyldimethylammonium chloride (PDAC, a strong polycation),poly allylamine hydrochloride (PAH, a weak polycation), sulfonatedpolystyrene (SPS, a strong polyanion), and poly acrylic acid (PAA, aweak polyanion). Examples of a single charge site bearing a multiplecharge include multiply charged metal ions, such as, without limitation,Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Cu²⁺, Cd²⁺, Sn⁴⁺, Eu³⁺, Tb³⁺, andthe like. Multiply charged metal ions are available as salts, e.g.chloride salts such as CoCl₂, FeCl₃, EuCl₃, TbCl₃, CdCl₂, and SnCl₄.

The film can include hydrogen bonding polymers, such as, for example,polyacrylamide (PAm), polyvinylpyrolidone (PVP), and polyvinyl alcohol(PVA). The light absorbing film can include more than two materials. Oneof these materials is the light absorbing material and one of the othermaterials is either a multivalent ionic species or hydrogen bondingpolymer. Additional materials may be included in the film to promotecrosslinking, adhesion, or to sensitize light emission or absorption.

The thin films can include one or several layers of a polyelectrolyteand one or more charged species with strong dipole-dipole interactionsand any additional dopants. At least one of the charged species used forstrong dipole-dipole interactions has a charge opposite that of thepolyelectrolyte used for the scaffold. When sequentially applied to asubstrate, the oppositely charged materials attract one another formingan electrostatic bilayer. The polyelectrolyte provides a scaffold forthe species with strong dipole-dipole interactions to form a layeredstructure. These films are compatible with other processes of buildingthin films through alternate adsorption of charged species. The filmscan be interspersed in a multifilm heterostructure with other thinfilms.

The charged species with strong dipole-dipole interactions can be asingle type of species, such as a single type of J-aggregating material(for example, a cyanine dye). Alternatively, several charged specieswith strong dipole-dipole interactions among the species could be used.The species used for the strong dipole-dipole interacting layer can haveindividual dipoles that can couple together to produce a coherentquantum mechanical state. This allows for the buildup of coherence intwo dimensions, producing effects in the probe dimension perpendicularto the interacting species.

J-aggregates of cyanine dyes have long been known for their strongfluorescence. This strong fluorescence makes J-aggregates a desirablecandidate for use in organic light-emitting devices (OLEDs), and suchdevices have been produced. The layer-by-layer (LBL) technique for filmgrowth, first developed by Decher et al., was extended to create thinfilms of J-aggregates, which have been used to create an OLED withJ-aggregates as emitters. See, for example, E. E. Jelley, Nature 1936,138, 1009; M. Era, C. Adachi, T. Tsutsui, S. Saito, Chem. Phys. Lett.1991, 178, 488; G. Decher, J. D. Hong, J. Schmitt, Thin Solid Films1992, 210, 831; H. Fukumoto, Y. Yonezawa, Thin Solid Films 1998, 329,748; S. Bourbon, M. Y. Gao, S. Kirstein, Synthetic Metals 1999, 101,152; Bradley, M. S. et al., Advanced Materials 2005, 17, 1881; and U.S.patent application Ser. No. 11/265,109, filed Nov. 3, 2005, each ofwhich is incorporated by reference in its entirety. J-aggregates (andthin films including J-aggregates) can have a high oscillator strengthat a characteristic wavelength. In other words, the J-aggregate stronglyabsorbs light of the characteristic wavelength. The characteristicwavelength depends primarily on the identity (i.e., the chemicalstructure) of the material forming the J-aggregate, and to a lesserdegree on other factors, such as the chemical environment of theJ-aggregate. For example, multilayer films of polycation and anionicJ-aggregate dye contain a high density of J-aggregate and therefore havea high peak absorption coefficient of α=1.0×10⁶ cm⁻¹. See M. S. Bradley,et al., Advanced Materials 17, 1881 (2005), which is incorporated byreference in its entirety. Because the film has a high absorptioncoefficient at its characteristic wavelength, a very thin film (e.g.,less than 50 nm thick, less than 25 nm thick, less than 10 nm thick, or5 nm thick or less) can absorb much of the tight of the characteristicwavelength.

Layer-by-layer (LBL) processing of polyelectrolyte multilayers canproduce conformal thin film coatings with molecular level control overfilm thickness and chemistry. Charged polyelectrolytes can be assembledin a layer-by-layer fashion. In other words, positively- andnegatively-charged polyelectrolytes can be alternately deposited on asubstrate. One method of depositing the polyelectrolytes is to contactthe substrate with an aqueous solution of polyelectrolyte at anappropriate pH. The pH can be chosen such that the polyelectrolyte ispartially or weakly charged.

A substrate subjected to sequential immersions in cationic and anionicsolutions (i.e., solutions of polycation and polyanion), or SICAS, canproduce a multilayer including a number of electrostatic bilayers on thesubstrate. An electrostatic bilayer is the structure formed by theordered application of a multiply charged species (e.g., apolyelectrolyte or metal ion) and an oppositely charged material (e.g.,a light absorbing material, polyelectrolyte or counterion). Theproperties of weakly charged polyelectrolytes can be preciselycontrolled by changes in pH. See, for example, G. Decher, Science 1997,277, 1232; Mendelsohn et al., Langmuir 2000, 16, 5017; Fery et al.,Langmuir 2001, 17, 3779; Shiratori et al., Macromolecules 2000, 33,4213, each of which is incorporated by reference in its entirety.

The process conditions used in the deposition of the film can be varied.Some process conditions that can be varied include concentration,temperature, pH, salt concentration, co-solvent, co-solventconcentration, and deposition time. The temperature can be variedbetween, for example, 0° C.. and 100° C.., or between 5° C.. and 80° C..The pH can be varied from 0.0 to 14.0, or from 3.0 to 13.0. The saltconcentration can range from deionized (i.e., no salt added) to 1 M.NaCl and KCl are examples of salts used. Solutions can be prepared usingwater as the sole solvent, or with water and a co-solvent, such as anorganic solvent. Some exemplary organic solvents include methanol,ethanol, isopropanol, acetone, acetic acid, THF, dioxane, DMF, andformamide. The deposition time can be 1 second or less; 30 seconds orless; 1 minute or less; 5 minutes or less; 10 minutes or less; 1 hour orless; or several hours or more. In some circumstances, deposition timeswill be in the range of 1 second to 10 minutes. Deposition of the filmcan include a post-treatment of the film. Post-treatment is anytreatment applied to the film after the last bilayer is applied. Thepost-treatment can include a heat treatment, a pH treatment, a chemicalmodification, or other treatment. The post-treatment can be selected toalter a desired property of the film, such as its mechanical stabilityor porosity.

The density of the film can be modified by repeatedly immersing thesubstrate into solutions of the light absorbing material prepared withdifferent process conditions. As an example, by cyclically immersinginto a solution held at a temperature of 20° C.. and then in a secondsolution held at 60° C.., the crystallinity of the resultant film can beenhanced and dye density increased compared to films not treated in thismanner.

The film can include a plurality of bilayers, such as fewer than 100,fewer than 50, fewer than 20, or fewer than 10 bilayers. The film caninclude 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5., 6.0,6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10.0 bilayers. In someembodiments, the film can include bilayers substantially free of lightabsorbing material, e.g., bilayers where one layer includes a polycationand the other layer includes a polyanion. Including bilayers that aresubstantially free of light absorbing material can be advantageous, forexample, in altering the adhesion of the film to a substrate or inaltering the thickness of the film.

The light absorbing film can be deposited on a hydrophilic orhydrophobic substrate. The film can be deposited onto conducting (e.g.,metallic), semiconducting, or insulating layers (including glass andplastic); or bio-compatible materials, examples of which are a polymerfilm that is hydrophilic or hydrophobic, an oxide layer, a metal oxidelayer, a metal layer, a DNA-coated surface, and others. Examples of ahydrophilic polymer layer include polyelectrolytes and hydrogen bondingpolymers; amino acids; proteins; and hydrophilic polymers. Examples ofhydrophobic polymers include PDMS, Poly-TPD, and MEH-PPV. Metal oxidelayers include, for example alumina, titania, and zinc oxide. Examplesof semiconducting layers are layers of Si, Ge, GaAs, GaN, AlGaAs, GaAsP,CdSe, CdS, ZnS, and metal halides, such as AgCl, AgBr, and AgI. Adhesionof the light absorbing film to the substrate can be promoted by varyingthe process conditions described above.

The light absorbing film can be optically coupled to a reflectiveelement to form a resonator. The reflective element can be a layer ofsemiconductor deposited on a glass substrate. Alternatively, thesubstrate itself can be a semiconductor substrate, for example, siliconas which is reflective at the wavelength of 1550 nm. A CCR can becontructed on top of a silicon substrate, possibly eliminating the needfor additional mirror layers.

An exemplary resonator structure that optically couples the absorbingfilm to a reflective element is illustrated in FIG. 1A. In FIG. 1A, anabsorbing film 2 is a high oscillator strength film, such as, forexample, an electrostatic multilayer of a J-aggregate forming dye and apolyelectrolyte. The absorbing film 2 has a defined thickness, referredto as d_(α). A surface of the absorbing film 2 is in opticalcommunication with a light transmitting medium 1, e.g., air. Anothersurface of the absorbing film 2 is in optical communication with areflective element 4 via an optically transmissive spacer layer 3. Thespacer layer 3 has a defined thickness, d_(s). The entire structure isarranged on a substrate 5.

The resonator of FIG. 1A can achieve critically coupled resonance whencertain conditions are met. Critically coupled resonance results innear-complete absorption of light of a particular wavelength by theresonator. The CCR structure allows near-complete absorption even whenthe absorbing film is very thin (e.g., less than 50 nm, less than 25 nm,or less than 10 nm). The CCR structure absorbs light (of the criticalwavelength) to a greater degree than the absorbing film does by itself(i.e., when the absorbing film is not optically coupled to a reflectiveelement). The CCR can have an effective absorption coefficient at acritical wavelength of greater than 10⁶ cm⁻¹, greater than 2×10⁶ cm⁻¹,greater than 3×10⁶ cm⁻¹, greater than 4×10⁶ cm⁻¹, greater than 5×10⁶cm⁻¹, or greater than 6×10⁶ cm⁻¹. The absorption coefficient a can berelated to the complex portion κ of the complex index of refraction bythe formula α=4πκ(λ)/λ, where λ is the wavelength of light incentimeters. As such, the CCR can have an extinction coefficient κ of atleast 1, at least 2, at least 3, at least 4, or at least 5.

The optical properties of the resonator illustrated in FIG. 1A can bedescribed mathematically. The mathematical description accounts for fourregions of different refractive index: air (1), absorbing film (2),spacer (3), and reflective element (4). For simplicity in themathematical description, reflective element 4 can be treated as a layerof silver mirror sufficiently thick to neglect reflections from themirror/substrate interface. The mathematical description includes threeinterfaces between materials of different refractive index: interface12, between air 1 and the absorbing film 2; interface 23, betweenabsorbing film 2 and spacer 3; and interface 34, between spacer 3 andreflective element 4. The thickness of the absorber layer (d_(α)) isrepresented by L₂, and the thickness of the spacer is represented by L₃.For normal-incident light (i.e., light approaching the absorbing film at90° to the surface of the film) of wavelength λ, the reflectioncoefficient r of the resonator is given by:

$r = \frac{\left\lbrack {{r_{12}\left( {1 + {r_{23}r_{34}^{2j\; \beta_{3}L_{3}}}} \right)} + {^{2j\; \beta_{2}L_{2}}\left( {r_{23} + {r_{34}^{2j\; \beta_{3}L_{3}}}} \right)}} \right\rbrack}{\left\lbrack {\left( {1 + {r_{23}r_{34}^{2j\; \beta_{3}L_{3}}}} \right) + {r_{12}{^{2j\; \beta_{2}L_{2}}\left( {r_{23} + {r_{34}^{2j\; \beta_{3}L_{3}}}} \right)}}} \right\rbrack}$

where the Fresnel coefficient r_(ij) for interface if is:

$r_{ij} = \frac{\left( {{\overset{\sim}{n}}_{i} - {\overset{\sim}{n}}_{j}} \right)}{\left( {{\overset{\sim}{n}}_{i} + {\overset{\sim}{n}}_{j}} \right)}$

and the wavevector β₁ for the i^(th) layer is:

$\beta_{i} = \frac{2\pi \; {\overset{\sim}{n}}_{i}}{\lambda}$

where ñ₁ is the complex index of refraction for the i^(th) layer.

The reflection coeffecient of the resonator is related to its percentreflectance (R) by R=|r|². Critical coupling occurs when the resonatorparameters are selected such that R=0% at λ_(e), since no light istransmitted through the CCR. In actual devices, R can approach 0% but begreater than 0%. For example the value of R at λ_(c) can be 10% or less,5% or less, or 2% or less. The reduction in reflectance is due tocritical coupling, and not to an antireflective material. Althoughantireflective materials are known, the CCR can be substantially free ofantireflective material. Critical coupling of the light absorbing filmand reflective element can occur at a wide range of temperatures, suchas 77 K or higher. The critical coupling can occur at any temperaturefrom 77 K and higher until the temperature becomes so high that thematerial of the CCR begins to degrade. Critical coupling can occur, inthe range of 250 K to 350 K, or 200 K to 400 K.

FIG. 1B illustrates a CCR in which the reflective element is adielectric Bragg reflector (DBR). The DBR includes alternating layers ofmaterial having different refractive indices, where the thickness ofeach layer (d_(l)) is chosen to meet the Bragg condition,d_(i)=λ/4n_(i), where λ is wavelength and n_(i) is the refractive indexof material i. The DBR can be made by alternately sputtering twodifferent materials of known refractive index to deposit alternatinglayers of a desired thickness on a substrate. The materials can be, forexample, metal oxides such as a titanium oxide or an aluminum oxide. Thematerials can be, for example, metal oxides such as a titanium oxide,hafnium oxide, silicon dioxide, zirconium oxide or an aluminum oxide.Other materials that can be used in the DBR are conductive metal oxidessuch as indium tin oxide, tin oxide, zinc oxide or indium zinc oxide.The DBR can also be made of polymers and or chalcogenide materials. Thelayers can be deposited using any one or a combination of a variety ofdeposition techniques that include, for example, sputter coating,thermal evaporation, chemical vapor deposition, electron beamevaporation, spin casting, and extruding.

A top coat can be included with the CCR. The CCR can include a mirrorelement, spacer element, and absorber layer, where the mirror layer wasdeposited on a substrate followed by deposition of the spacer layer andabsorber layer. In certain structures, these three elements can beintegrated with other layers of materials. For instance, on top of theabsorber layer a protective coating can be placed on top of atransparent material, such as an organic small molecule or polymer ormetal-oxide like silica or alumina, and critical coupling can beachieved. Furthermore, a CCR can be constructed where the absorber layercan be deposited onto the substrate, followed by spacer and mirrorlayers.

An optical switch can include a CCr. When light is incident on the CCRfrom the absorber side of the device, almost none of the light isreflected, and is instead absorbed within the absorber layer. However,the absorptive tendencies of the absorber layer can be momentarilyswitched off. During this brief moment, in which the absorber layer istemporarily bleached, the CCR reflects the light incident on it from theabsorber layer side of the device. It is possible that briefly themirror layer can reflect light as efficiently as if the absorber wasphysically not present. The temporary bleaching of the absorber layercan be accomplished by optically exciting the absorber layer with a highpower burst of light energy. The burst of light energy can be deliveredat the critical wavelength at which the CCR can act or at anotherwavelength at which the absorber layer can absorb light. In the case ofJ-aggregates of cyanine dyes, the extremely large absorption non-linearresponse coefficient (χ⁽³⁾) at the J-aggregate resonance due to thecooperative coupling amongst the individual dye molecules can enableoptical switching to be achieved with very low energy, arguably lessenergy than is required to switch state-of-the-art silicon transistors.The switching recovery time can occur within less than 20 ps, forexample, within 3 ps, which can result in a fast all-opticalroom-temperature optical switch architecture which can find use in shorthaul optical interconnects on silicon microchips.

A light harvesting device can include a CCR. The CCR structure caninclude mirror element, spacer and absorber layer with electricalcontacts in order to extract photovoltaic energy from the light absorbedwithin the absorber layer. The electrical contacts can be placed on topof the absorber layer, underneath it, or on the sides adjacent to it.The metallic mirror layer can act as one of these contacts. A protectivelayer deposited on top of the absorber layer, if made of an electricalconductive material, can also be one of the electrical contacts. Forexample, the contact can include indium tin oxide.

A chemical sensor can include a CCR. There are at least two methods inwhich the CCR can be utilized for chemical sensing applications. Thefirst method can be a reflectance/absorbance based sensor. The secondmethod is a fluorescence based sensor. In the first method, a componentthat is chemically sensitive to the chemical to be sensed can beincorporated into the absorber layer element of the CCR. When thechemical is present, this component can alter the absorber layer'sability to respond to light at the CCR critical wavelength. Thereflectance of the CCR can thus be modulated from nearly 0% (chemicalabsent) to nearly 100% (chemical present). If the absorber layermaterial itself is sensitive to this chemical, then adding a compoundsto the absorber layer would not be necessary although it could stillenhance sensitivity. In the second method, the absorber layer can absorbnearly all of the light incident at the critical wavelength. Thisabsorbed light energy would actually be emitted as fluorescence from theabsorber material itself or from another component incorporated in thelayer to accept the absorbed energy and radiate it. However, in thepresence of the chemical to be sensed, one of these fluorescent pathwayscan be quenched. Thus the presence or absence of fluorescence would beindicative of the absence or presence of the chemical in theenvironment. Another component can be incorporated with the absorberlayer that is sensitive to the chemical to be sensed.

EXAMPLES

In the CCR of FIG. 1B, the absorbing layer was a thin film consisting oflayers of the cationic polyelectrolyte PDAC (polydiallyldimethylammoniurn chloride) and J-aggregates of the anioniccyanine dye TDBC(5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl)benzimidazoliumhydroxide, inner salt, sodium salt). Molecular structures of PDAC andTDBC are shown in FIG. 1B. The DBR was 8.5 pairs of sputter-coated TiO₂and Al₂O₃ layers, ending on TiO₂. The spacer layer was an additionalsputter-coated layer of Al₂O₃. The J-aggregate layer was prepared bydepositing PDAC and TDBC by sequential immersion into cationic andanionic aqueous solutions (pH=5.5) utilizing the technique described inM. S. Bradley, et al., Advanced Materials 17, 1881 (2005), which isincorporated by reference in its entirety. Reflection and transmissionmeasurements were made with light incident from the I-aggregate side ofthe device.

The critically coupled resonator (CCR) structure can include adielectric Bragg reflector (DBR), a transparent spacer layer, and alayer of J-aggregate cyanine dye. The DBR can include 8.5 pairs ofsputter coated TiO₂ and Al₂O₃ layers, ending on TiO₂. The spacer layercan be an additional sputter coated layer of Al₂O₃. The J-aggregatelayer can include of the cationic polyelectrolyte, PDAC, and the anioniccyanine dye, TDBC, deposited via sequential immersion into cationic andanionic aqueous solutions (pH=5.5) utilizing a previously describedtechnique (see, M. S. Bradley, J. R. Tischler, V. Bulovic, Adv. Mater.2005, 17, 1881, which is incorporated by reference in its entirety).

When constructing a system for demonstrating strong coupling, thenatural tendency is to focus primarily on the optical properties of thecomponents, i.e. the excitonic layer and the microcavity, and on thefully integrated composite system to check for Rabi-splitting.Understandably, optical measurements of the half cavity structureconsisting of just one of the two mirrors from the microcavity and theexcitonic layer are routinely not reported.

FIG. 2 presents reflectance and transmittance data for the CCR, alongwith reflectance data for the neat PDAC/TDBC film and for the dielectricstack of DBR with spacer layer without an absorbing film applied. Thevertical axis in FIG. 2 runs from 0.0 to 1.0, which corresponds to 0% to100% in reflectance and transmittance. When light of wavelengthλ_(c)=591 nm was incident on the CCR of FIG. 1B from the absorbing layerside of the device, the measured reflectance was R=2% (FIG. 2). Incontrast, for the DBR with spacer but without the absorbing layer, thereflectivity at λ_(c)=591 nm exceeded 95%, showing the dramatic changein reflectance due to critical coupling. For the same CCR, thetransmittance at λ_(c) is T=1%. Consequently, 97% of the incident lightwas absorbed within the 5.1±0.5 rim thick absorber layer, yielding amaximum effective absorption coefficient of α_(eff)=6.9×10⁶ cm⁻¹.

A wavelength resolved T-matrix simulation (FIGS. 3 and 4) numericallyconfirmed the critical coupling phenomenon. To simulate the CCR'sreflectance, T-matrices corresponding to the PDAC/TDBC film and the DBRwere constructed. The film was modeled following the procedure describedin M. S. Bradley, et al., Advanced Materials 17, 1881 (2005), wherein(n,κ) are obtained through a Kramers-Kronig regression based onreflectance data of the neat film deposited on a SiO₂ substrate (FIG.3). FIG. 3 displays spectrally resolved real and imaginary components ofthe refractive index, (n,κ), for 5.1±0.5 nm thick PDAC/TDBC filmdeposited on an SiO₂ substrate.

In modeling the CCR, the DBR was modeled as 8.5 pairs of TiO₂ and Al₂O₃layers, with refractive indices of n=2.39 and n=1.62 respectively, withlayer thickness adjusted to satisfy the Bragg condition (d_(i)=λ/4n_(t))for λ=565 nm, to reflect the results of the experiment. The maximumvalue of κ occurred at λ=593 nm, while the peak reflectance of the thinfilm occurs at λ=595 nm. These models were combined with a model of thespacer layer with n_(s)=1.62 and thickness, d_(s), left as a freeparameter. The simulation reproduced critical coupling at λ_(c)=591 nm(FIG. 4) for a spacer layer thickness d_(s)=90 nm and odd multiplesthereof. FIG. 4 compares measured and calculated reflectance for the CCRdevice and DBR spacer stack. The calculated reflectance was based on theT-matrix formalism. The calculated fit matched the experimentallyobserved reflectance minimum at λ_(c)=591 nm.

When d_(s) was set to a value greater or less than 90 nm, criticalcoupling did not occur at another wavelength, because once the absorberlayer thickness, d_(o), was set, critical coupling can occur only at onespecific wavelength, λ_(c). The simulation also predicted that criticalcoupling was achieved at λ_(c)=591 nm with an Ag mirror replacing theDBR, and d_(s)=90 nm was still the critical thickness. With a non-idealmetallic mirror (e.g. Ag), not all of the light absorption was predictedto occur in the absorber layer, as would be the case with an ideal metalmirror (κ→∞), but the reflectance from top surface of the CCR was stillpredicted to be zero, as a result of critical coupling.

The critical coupling phenomenon observed for the 5.1 nm thick film ofPDAC/TDBC spaced 90 nm from the DBR of FIG. 1B is not limited to theseparticular materials and thicknesses. Critical coupling can be achievedwith any thin film absorber layer of sufficient oscillator strength(i.e., κ), providing that d_(α)and d_(s) are set to the appropriatethicknesses. To demonstrate this, a generalized formalism of criticalcoupling for the CCR structure of FIG. 1B was constructed. As describedabove, critical coupling occurs when the resonator parameters areselected such that R=|r|²=0%, where

$r = \frac{\left\lbrack {{r_{12}\left( {1 + {r_{23}r_{34}^{2j\; \beta_{3}L_{3}}}} \right)} + {^{2j\; \beta_{2}L_{2}}\left( {r_{23} + {r_{34}^{2j\; \beta_{3}L_{3}}}} \right)}} \right\rbrack}{\left\lbrack {\left( {1 + {r_{23}r_{34}^{2j\; \beta_{3}L_{3}}}} \right) + {r_{12}{^{2j\; \beta_{2}L_{2}}\left( {r_{23} + {r_{34}^{2j\; \beta_{3}L_{3}}}} \right)}}} \right\rbrack}$

where the Fresnel coefficient r_(ij) for interface ij is:

$r_{ij} = \frac{\left( {{\overset{\sim}{n}}_{i} - {\overset{\sim}{n}}_{j}} \right)}{\left( {{\overset{\sim}{n}}_{i} + {\overset{\sim}{n}}_{j}} \right)}$

and the wavevector β_(i) for the i^(th) layer is:

$\beta_{i} = \frac{2\pi \; {\overset{\sim}{n}}_{i}}{\lambda}$

FIG. 5 illustrates the generalized formalism for critically couplingabsorber layer of FIG. 1B as a function of absorber κ. Thicknesses forabsorber and spacer layers were normalized to the CCR wavelength, λ_(c).The reflectance plotted was at λ_(c). FIG. 5 shows the absorber andspacer layer thicknesses required to achieve critical coupling as afunction of the absorber layer oscillator strength κ. The result wasplotted for three different values of the real part of the absorberlayer refractive index, n_(α)∈(1.55, 1.75, 2.0.), n_(s)=1.7 throughout,and the single mirror layer was assumed to be Ag, with complexrefractive index ñ=0.259±j3.887 at λ=591 rim (this value of ñ wasderived from a fit of published ñ values, see H. J. Hagemann, et al., J.Opt. Soc. Am. 65, 742 (1975), which is incorporated by reference in itsentirety). The thicknesses are normalized to λ_(c) to emphasize thegenerality of this model. The model showed that to satisfy CCRconditions as κ increases, the absorber layer thickness must decrease,with a corresponding increase in the spacer layer thickness. The modelalso showed that for a given κ, as n_(α)increases, d_(s) decreases, asexpected, while d_(α)stayed relatively constant. The model dictates thatin order to critically couple the d_(α)=5.1 nm thick PDAC/TDBC film ofFIG. 1 at λ_(c)=584 nm (d_(α)/λ_(c)=0.87%) the extinction coefficient ofthe film must be κ=4.2, which also sets d_(s)/λ_(c)=15% or equivalentlyd_(s)=88 nm for n_(α)=2.0 and n_(s)=1.7. These theoretical values agreewell with the experimentally measured κ=4.2 and n_(α)=2.1 at 591 nm(from FIGS. 3) and d_(s)=90 nm for the CCR structure in FIG. 2 withn_(s)=1.62.

For the 5.1 nm thick film of PDAC/TDBC, the results in FIG. 5 were inclose agreement with the experimental results and with the fullsimulation for the CCR structure (FIG. 4). For wavelengths in the rangearound λ_(c)=584 nm and for d_(α)=5.1 nm, d_(α)/λ_(c)=0.0087, whichdictates that κ=4.2. From the dispersion relation of FIG. 3, the valueof κ=4.2 corresponds to n_(α)=2.1 and the observed spacer layerthickness is d_(s)=90±1 nm. Consistent with these observations, forn_(α)=2.0, FIG. 5 sets ds/λc=0.147, or equivalently d_(s)=90.3 nm forλ_(c)=584 nm.

Table 1 presents calculated critical thicknesses of absorber and spacerlayers for critically coupling a thin film with absorption coefficientα=4.0×10⁵ cm⁻¹. Real component of absorber layer refractive indexn=1.75. Average reflectance values were 0.3±0.1%. The critical thicknessin absolute terms was relatively constant across wavelengths for fixedα.

TABLE 1 Wavelength λ_(c) (nm) κ d_(a) (nm) d_(a) (%) d_(s) (nm) d_(s)(%) 350 1.11 12.4 3.5 27.6 7.9 410 1.31 12.1 3.0 36.4 8.9 450 1.43 12.22.7 42.1 9.4 525 1.67 12.1 2.3 53.2 10.1 600 1.91 12.0 2.0 65.0 10.8 7002.15 12.0 1.8 77.4 11.5

Table 2 presents calculated critical thicknesses of absorber and spacerlayers for critically coupling a thin film of absorption coefficient κto either of two wavelengths. The real component of absorber layerrefractive index was n=1.75. Average reflectance value was 0.6±0.3%, andthe same reflectance was produced at both critical wavelengths.

TABLE 2 Wavelength λ_(c) (nm) κ d_(a) (%) d_(a) (nm) d_(s) (%) d_(s)(nm) 410 0.93 5.0 20.4 7.6 31.2 584 0.93 5.0 29.1 7.6 44.4 410 2.79 1.56.3 13.5 55.2 584 2.79 1.5 9.0 13.5 78.7 410 4.65 0.9 3.6 16.0 65.6 5844.65 0.9 5.2 16.0 93.5

If the thin films that produce critical coupling in the CCR weredeposited as neat films on glass (n=1.48), T-matrix simulation showedthat they would absorb, on average, 32% of incident light at λ_(c). Theabsorption increase due to the critical coupling was therefore a factorof 3.1. Similarly, if such an absorber layer were inserted into asymmetric λ/2 microcavity, the maximum absorption would be 50% (withT=25% and R=25%). Thus, the CCR structure was not only convenient (i.e.,easier to make than a symmetric microcavity), it was also essential formaximizing light absorption. This suggests several practical deviceimplementations. For example, the CCR structure can be used in opticallystimulated chemical sensors where a thin luminescent chemosensitive filmis deposited on top of the CCR structure and excited by energy transferfrom the CCR absorber layer. See, for example, A. Rose, et al., Nature434, 876 (2005), which is incorporated by reference in its entirety.Compared to existing structures, a factor of 6 reduction in pump poweris expected, since the chemosensitive films would effectively absorb 3.2times more light, and the back mirror would direct twice thephotolumuniscence into the detector. Application of the CCR phenomenoncan also facilitate development of single photon optics where it isdesirable to absorb a photon with 100% probability in the thinnestpossible films.

When a thin film of PDAC/TDBC is placed at the antinode of the opticalfield of a microcavity resonantly tuned to the excitonic absorptionpeak, strong coupling was observed. See J. R. Tischler, et al., Phys.Rev. Lett. 95, 036401 (2005), which is incorporated by reference in itsentirety. New polaritonic resonances appeared in the linear opticalmeasurements of the composite structure corresponding to thesuperposition states of the strongly coupled light/matter system. Strongcoupling was achieved even in low Q all metal microcavities, with Rabisplitting of 265 meV, due to the high absorption coefficient of thePDAC/TDBC films and their relatively narrow reflectance linewidth,FWHM=67 meV. The polaritonic band gap observed in these full microcavitystructures was manifested as a high reflectance at the uncoupledexcitonic resonance. The appearance of a polaritonic band gap followsnaturally from the realization that the strongly coupledexciton-polariton device is a CCR plus a “top” DBR, separatedapproximately λ/4n away from the CCR absorbing layer, where n is therefractive index of the transparent spacer layer. With the CCR in place,when the “top” DBR is added to complete the microcavity, there is nooptical feedback to cause a resonant dip in reflectance and thereforethe high reflectance of the “top” DBR is observed as the polaritonbandgap.

As alternatives to PDAC/TDBC and other J aggregates, CCR's can also beconstructed with a variety of highly absorptive materials. Amongnon-epitaxially grown materials, CCR structures could be implementedwith organic polymers that are used in biological assays and chemicalsensors, with molecular materials that are used in photodetectors andxerographic photoresistors, and in the emerging uses of colloidallygrown inorganic nanocrystal quantum dots (QDs), with the QD continuumstates providing the necessary absorption. See, for example, C. A.Leatherdale, Woo, F. V. Mikulec, M. G. Bawendi, J. Phys. Chem. B 106,7619 (2002), which is incorporated by reference in its entirety. A CCRcan be used, for example, in chemical sensors, nanoscale thin-filmphotodetectors, or “Exciton-Polaritons” materials satisfying CCRcriteria.

Referring to FIGS. 6A-6C, a device including a critically coupledresonator can include a blue- or ultraviolet-light emitting structure,the light from which interacts with an absorber or emitter layer whichin turn transmits light at a plurality of selected wavelengths through acritically coupled resonator. Any combination of the absorber or emitterlayer or the DBR layer can be patterned.

Photoluminescence can be increased by a factor of about 6 when acritically coupled resonator structure is optically excited near λ_(c).A factor of 3 increase can be attributed to the critically coupledresonator film, which absorbs 32% when on glass and nearly 100% on DBR,and a factor of 2 collection efficiency can be attributed to acollection efficiency boost due to a back DBR reflector. Using this typeof structure, a sensor can be developed that can be optically pumpedwith ⅙^(th) power. See FIG. 7.

Other embodiments are within the scope of the following claims.

1.-50. (canceled)
 51. A device including: a light emitting layerconfigured light of a first wavelength; an absorber or emitter layerconfigured to receive the first wavelength emitted from the lightemitting layer and transmit light of a second wavelength; and adielectric Bragg reflector configured to receive the light of the secondwavelength and transmit light out of the device.
 52. The device of claim50, wherein the light emitting layer is patterned.
 53. The device ofclaim 50, wherein the absorber or emitter layer is patterned.
 54. Thedevice of claim 51, wherein the light emitting layer is patterned andthe absorber or emitter layer is patterned. 55.-57. (canceled)